As demand for electric energy has risen in recent years, in some population centers such as New York City, the difference between the peak electric power that can be supplied and the peak demand for that electric power, referred to as “margin”, has narrowed to the point that a severe anomaly could eliminate that margin altogether. For example, a sustained heat wave in New York City can erode that margin to the point that “brown-outs” occur, and if the supply of electric power cannot be increased, or demand outstrips the increased supply, “black-outs” can occur. To reduce the risk of “brown-outs” and “black-outs”, responsible power authorities have tried to locate sufficient power generation capacity in or near population centers to meet normal electrical demand, and to provide for sufficient transmission grid capacity serving such areas so that in the event a severe anomaly such as a heat wave does occur, additional electrical energy from distant power plants can be provided to avoid “brown-outs” or “black-outs”, albeit at a substantial cost due to the losses associated with transmission of electricity over long distances and at high load factors. Unfortunately, in times of extreme demand, the transmission grid has a limited capacity to provide additional electric power to population centers such as New York City, and despite years of discussion and a general acceptance of the need to upgrade the transmission grid, if and when additional capacity will be added remains unclear. In the absence of sufficient transmission grid capacity to meet severe anomalies, population centers such as New York City must rely on locally generated electrical power to meet that demand, thereby avoiding the need to bring that additional electrical power in over the transmission grid.
While “base-load” generating capacity may be provided by coal-fired power plants, nuclear power plants, or “combined cycle” gas turbines, generating capacity to meet peak demand is often provided by “simple cycle” gas turbines. As those skilled in the art will readily appreciate, simple cycle gas turbines produce substantial amounts of secondary heat, which is typically exhausted to the atmosphere. Combined cycle gas turbines also produce secondary heat, although to a lesser extent. As used herein, the term “secondary heat” means heat produced 1) as, or by, a byproduct of the machines, electrical equipment, or industrial or biological processes that produce it, or 2) as a useful product in machines, electrical equipment, or industrial or biological processes but which can also be used for a secondary purpose without impairing the function of such machines, electrical equipment, or industrial or biological processes.
Unfortunately, increasing local electrical power generation capacity by installing additional gas turbines to meet demand in times of severe anomalies faces substantial obstacles, the most obvious being cost. While the costs associated with installing “base-load” generation can be recouped by generating and selling electrical power 24 hours a day throughout the year, the costs associated with installing generating capacity to meet peak demand (so called “peakers”) must be recouped over a few hours of operation per day during a few months of the year. Since this results in very high costs per megawatt hour of electrical power actually produced, power authorities are understandably hesitant to invest in additional peakers where they have other options.
Space is also an issue in population centers such as New York City, where real estate is at a premium. Acquiring sufficient land to build a new power plant while accommodating nearby residential and business interests can delay construction of additional power generation capacity long past the date that additional capacity is needed. Permitting requirements to achieve acceptable emissions levels can further delay construction of additional generating capacity, particularly in densely populated areas.
Faced with such cost, space and emissions obstacles, power authorities often resort to the use of existing, less efficient generating capacity. Since this generating capacity is already installed, costs and real estate are not an issue, and in some cases this generating capacity may be “grand-fathered” in under existing permits. Also, older gas turbines are generally not as efficient as newer ones, which translates to more waste heat being given off per megawatt hour being generated. As a result, use of this generating capacity often produces higher emissions, of carbon monoxide and/or other pollutants such as NOx and ozone, per megawatt hour of power generated than their “base-load” counterparts, which can raise air quality concerns depending upon what time of day this generating capacity is operated.
Compressed air energy storage (“CAES”) plants have been considered as a potential solution to deal with peak electrical power demands in population centers. In general, these systems store compressed air using off-peak electrical power, and then use that compressed air to produce electrical power at times of peak demand. The air storage for CAES systems has traditionally been large underground caverns that were pressurized to a maximum pressure, and then bled down until they reached the minimum to the operating pressure of the expanders, at which time the expansion cycle was stopped until the compression cycle was run again. This type of cyclic duty results in a compression system where all of the stages of the compressor are always engaged and have a relative constant exit condition. For example, Dresser-Rand's “SmartCAES” system, which is shown schematically in FIGS. 1A and 1B, uses single or multiple multi-stage intercooled compressors 101A, 101B, 101C, 101D, 101E, 101F, 101G to produce compressed air that is stored in an air storage cavern 107, and multi-stage expanders 102A, 102B, 102C, 102D, 102E, 102F to expand the compressed air, in which the compression and expansion equipment are separate systems. In typical CAES systems, a recuperator 108 is used to transfer heat from the exhaust gas 103 of an expander 102F to the compressed air as it exits the air storage cavern 107. The recuperator 108 preheats the compressed air before it enters a dedicated high pressure combustor 106. There, fuel 110 is added and combusted, to further heat the compressed air before it is fed into a high pressure expander 102A. The compressed air exiting the high pressure expander 102C is then reheated in a low pressure combustor 109, where more fuel 112 is added and combusted, prior to being fed into a low pressure expander 102D. The electrical power produced by the expanders of the SmartCAES is normally the product of diffusion combustion in the high and low pressure combustors 106, 109, with relatively high emissions which require a selective catalytic reducer (“SCR”) (114) to meet common emission requirements (although some may now be operating with premixed combustion systems). In addition, since the SmartCAES systems are designed to have either 110 MW or 135 MW plant output ratings, either large natural caverns 107 must be available near the site of each SmartCAES installation, or the geology at the SmartCAES plant site must be suited for development of a cavern with the required characteristics. Therefore, locations for use of the SmartCAES system are limited by local geological conditions.
FIGS. 2A and 2B schematically show Energy Storage & Power's “CAES2” system, in which multiple multi-stage intercooled compressors 201A, 201B, 201C, 201D, 201E, 201F, 201G are used to produce compressed air that is stored in long air storage pipes, porous geological media or caverns 208. In the CAES2 system, a recuperator 206 is used to transfer heat from the exhaust gas of a gas turbine 207 to the compressed air prior to entering multi-stage expanders 202A, 202B, 202C, 202D, 202E, 202F. Depending on the particular system, partially expanded compressed air 203, specifically limited to that which can be bled from between the first and second expander stages 202A, 202B, may be taken off and delivered to the gas turbine's combustor 204 for power augmentation, or the compressed air may be expanded through all stages of the expanders 202A, 202B, 202C, 202D, 202E, 202F and then delivered directly to the inlet 205 of the gas turbine 207 as chilled air for power augmentation, provided that the air is below ambient temperature so as to be suitable for inlet cooling (see U.S. Pat. Nos. 5,934,063, 6,038,849, 6,134,893, 6,244,037, 6,305,158). These patented systems have recuperators 206, to preheat the air before it enters the expander 202A, and as a result, very hot compressed air is fed into the first stage of the expander 202A, often within 50° F. of the temperature of the gas turbine's exhaust gas. This very hot, compressed air is then expanded to ambient pressure in a manner such that the temperature of the compressed air does not go below freezing, to prevent icing issues in either the expander stages, or the gas turbine inlet. About 40% of the rated power output of a CAES2 plant is produced by the gas turbine 207 that is included with the system, and the remaining power comes from the generator 209 that is driven by the expanders 202A, 202B, 202C, 202D, 202E, 202F. The emissions of the CAES2 system are typically lower than those produced by the SmartCAES system due to incorporation of a gas turbine having a premix combustion system, and although the use of pipes for air storage 208 can allow the CAES2 system to be installed at locations that do not have existing caverns, the quantity of pipes needed by the CAES2 system increases the cost of installing a CAES2 system to the point that many power authorities may feel is prohibitive.
What is needed is a means of providing additional local electrical power during peak demand periods which does not necessitate the purchase of substantial quantities of additional land, can meet existing emissions requirements, has a high “round trip electrical efficiency” (i.e. energy output/energy input) and is cost-competitive as compared with other options.